This invention relates to web guide devices which control the lateral position of a web, which web is continuously carried in the longitudinal direction thereof, and, more particularly, to a web guide device employable in a web carrying line having a means for floatingly supporting the web by fluid cushion and a means for supporting the web directly on a solid surface, such as that of a roller.
The term "web" as used herein is intended to mean a belt-shaped material which is relatively flexible and somewhat rigid and is, generally, 5.mu.-1000 .mu. in thickness and 0.3 m-5 m in width, and which is made of plastics, such as polyethylene terephthalate, polypropylene, cellulose triacetate, vinyl polychloride, polycarbonate, etc., or metals, such as aluminum, copper, etc., papers, such as machine-made paper, synthetic paper, etc., or fabric, such as non-woven fabric. Depending on the intended use, the belt-shaped material may be covered with a film.
In web carrying lines which carry a web continuously in its longitudinal direction, it is often necessary to carry the web without causing the surface of the web to touch any of the solid surfaces, such as those of a pass roller, a drum, and canvas felt. In order to meet this requirement, a variety of means have been put into use in which a web is floatingly supported by utilizing a fluid cushion system in which a pressurized fluid, such as air or nitrogen gas, is jetted towards the surface of the web.
It is possible, with the proper arrangement of chambers adapted to jet the pressurized fluid, for the above-described fluid cushion supporting means to floatingly support the web along a web path in the form of an arch or a zigzag line extended in the longitudinal direction of the web. In this case, the frictional force in the lateral direction of the web supported by the floating support means is considerably smaller than that of the web supported directly by a solid support means. Accordingly, because of the lateral elongation of the web, the uneven web tension distribution in the lateral direction, and the uneven fluid cushion pressure in the lateral direction, the web is liable to laterally displace, while the web floating height is also liable to become uneven, and, therefore, a web supporting surface much wider than the width of the web is necessary. At worst, the web is brought into contact with the solid surfaces of the chambers or is greatly shifted from its predetermined running position, which stops the web from running.
In order to eliminate the above-described difficulties, a web guide device as disclosed in U.S. Pat. No. 3,826,416 has been provided, in which a web which has passed through the fluid cushion support means is brought into contact with the solid surface of a control roller to thereby turn the running direction of the web, and the control roller is swung around an axis perpendicular to the surface of the web whose running direction has been turned to thereby exert a higher tension on one of the edges of the web; that is, the web is displaced toward the edge having the higher tension in accordance with well-known principles to thereby correct the positional deviation of the web in the lateral direction.
The above-described web guide device can quickly control the position of the web in the fluid cushion support means with high accuracy, which otherwise is liable to be shifted in the lateral direction, and this contributes considerably to the improvement of the web carrying efficiency and characteristics.
There are a variety of different webs which are to be carried by the web carrying line. For instance, if the thickness of a web is relatively thin--that is, the web is relatively deformable--the formation of creases or wrinkles which may permanently deform the web and the tear of the margin of the web are frequently caused by the control roller of a conventional web guide device.
In addition, if the pressure sensitivity of the film provided on the web is high, the quality of the film in the vicinity of one of the edges of the web which is brought into contact with the control roller is considerably lowered.
As a result of investigation and analysis on the causes of the above-described difficulties, the inventors have found that a web tension distribution extremely biased toward one edge of the web is caused is generated by the above described swing action of the conventional roller system, and, therefore, the web is slidingly displaced on the surface of the control roller toward the web edge where the tension is higher. This occurs at a relatively high speed, as a result of which a web having a high rigidity is liable to crease and, especially in the vicinity of the edge where the highest tension is provided, cutting of the margin and breaking of the web are liable to occur. Furthermore, the film may also be broken by the pressure.
As a result of various studies and experiments on the control roller, the inventors have invented and put into use a novel web guide device.